The mid 90's was the era of Yahoo!'s directory. The web was organized by hierchical categories (the names and children of which were decided by Yahoo) and somebody visited sites to place them in a category. You searched the hierarchy, not the text of the web pages themselves. Search engines came only a little bit later).
Web browsers usually have the ability to save web addresses as bookmarks. These can also be organized, like a file system, or like the file cabinet each draws their metaphor from, in a system of hierarchical folders.
Web 2.0 has popularized the concept of tagging data; marking it rather than filing it. This increases the complexity of categorization exponentially.